Mythra is a Blade, which are personifications of weapons used by Drivers in the world of Alrest. Blades are essentially immortal but will revert to their Core Crystals if their Driver that awakens them dies. Mythra specifically is one of two legendary Blades known as the Aegis. She was first awakened by Addam, a central character of Xenoblade Chronicles 2 ~ The Golden Country, and later Rex, the protagonist of Xenoblade Chronicles 2. She forms a special bond with Rex and shares half of her life-force with him; if one were to die, the other would as well. Mythra has a variety of light-based powers, being only one of five other Blades in Alrest capable of harnessing Light energy; she also has control over Artifice Siren, a large mech floating in the skies above Alrest that is capable of launching immensely powerful blasts of light energy.
Around the year 3564, a priest named Amalthus climbed the World Tree, stole the two Aegis Core Crystals from Elysium, and brought them back to Alrest. Amalthus awakened the other Aegis named Malos, and Malos begins an attack on Alrest. To stop him, Amalthus approached Addam, prince of the Kingdom of Torna, and asked him to awaken the other Aegis core. Addam accepts and awakens the other Aegis, Mythra. The two would battle against Malos in a series of events that would come to be known as the Aegis War. The war came to a head in a battle where Malos forces Mythra to unleash her full power, which Addam is unable to control.
The battle culminates as Mythra and Malos face off in their own Sirens, causing massive destruction. At the end, Malos is assumed to have died in a fire, but the victory comes at a cost: three Titans were sunken as a result of the battles between Malos and Mythra. The final battle destroys the Kingdom of Torna, thereby killing thousands of people including many of Mythra's companions. Seeing the aftermath of her actions, Mythra spontaneously transforms into Pyra in anguish, splitting her personality. Seeing no other way to protect Mythra, Addam puts Pyra in stasis inside the Ancient Ship and sinks it under the Cloud Sea until someone that is able to control her comes along. He also seals away Mythra's true Aegis Sword deep inside Spirit Crucible Elpys to ensure only one who is worthy can wield that power.
500 years later, Pyra is reawakened by Rex; Pyra offers to give half of her life force to Rex after he was stabbed through the heart if he can take her to Elysium. Mythra leaves her duties as an Aegis to Pyra, as she is fearful of using her powers again. She only forcibly awakens after Rex and Pyra have a near death experience at the hands of Malos and Akhos, members of a new terrorist organization bearing the name of Torna. Malos survived from the Aegis War, albeit without many of his Aegis powers. Mythra scolds Rex for awakening her, but Rex claims he was just trying to defend her. After a while of reluctantly allowing herself to be used in battles, Mythra becomes more open and willing to work with Rex, and she eventually enables herself to swap with Pyra during battles. At multiple points in the story, Mythra notes that Rex is very similar to Addam, and she gradually opens up about her past.
Along the journey, Mythra reunites with Amalthus, who offers to help them reach Elysium. Along the way, Torna intercepts them multiple times. Jin, the leader of Torna and former ally to Addam and Mythra during the Aegis War, eventually overwhelms Mythra and threatens to kill Rex. Before he does so, Pyra offers herself over to Torna to ensure the safety of Rex and the other party members. Though faced with a setback, Rex and companions are informed of the true Aegis Sword inside Spirit Crucible Elpys, and they travel through the Spirit Crucible in hopes of finding the sword. At the end of the dungeon, they are able to find the sword, but it disintegrates in Rex's hand. Rex, now knowing what he must do, is unfazed and leads the party to where Pyra was taken.
It is revealed that Malos stole data from Pyra and Mythra's Core Crystal to restore his own powers, rendering Pyra akin to a corpse. In the ensuing confrontation, Pyra and Mythra reveal that, due to the guilt over Mythra's actions, they wanted to travel to Elysium to beg the Architect to let them die; Rex assures them that they do have a place in this world and that he will continue to protect them. In doing so, Rex summons the third Aegis Sword and awakens Pyra and Mythra's true form, Pneuma. With Pneuma's newfound powers, the party easily dispatches Jin and Malos.
After the battle, Rex, Mythra, and the others begin climbing the World Tree. They face multiple challenges on the journey up, including battles with Jin and later Amalthus. With Pneuma's newfound powers and Rex's ability as a true Aegis Driver, they are able to surpass these roadblocks and make it to Elysium. After meeting the Architect, Rex, Mythra, and the others face one final battle with Malos, who seeks to destroy Alrest once and for all. Rex and Pneuma eventually deal a fatal blow to Malos, ending the conflict. However, the World Tree begins to collapse, and Pneuma offers to stay behind in order to destroy the World Tree before it crashes down and destroys Alrest, much to Rex's objection. She leaves Rex with her Core Crystal, thereby restoring his heart, and the party escapes as Pneuma self-destructs the World Tree. After the credits, Pneuma's Aegis Core Crystal begins to glow again, and Pyra and Mythra return, now in separate bodies although it is left ambiguous as to whether they remember Rex or have been reborn without memories as Blades usually are. The story ends with Pyra and Mythra rejoining the group as they travel towards an island that the Titans merge into to create a new, expansive landmass.
As a playable character
Main article: Mythra (SSBU)
During Ultimate's development, Masahiro Sakurai considered adding characters from ARMS and Xenoblade Chronicles 2, however as their games were announced after the roster was completed, they were not included as fighters in the base game and Sakurai chose to have Mythra be a spirit instead. Three years later, Mythra would be announced as the tenth DLC fighter alongside Pyra in the February 17, 2021 Nintendo Direct. A character showcase was streamed March 4th, 2021 (March 5th, 2021 in some time zones), with Mythra being released later that same day.
Prior to the release of the Pyra/Mythra fighters, Mythra appeared as a Legend-class support spirit.
In Adventure Mode: World of Light, Mythra is presumably one of the countless spirits captured by Galeem during his takeover of the universe. After Galeem's defeat, Dharkon takes control of Galeem's spirits, including Mythra. After Dharkon's defeat, he flees to the final world, with Galeem following soon after. In the final world, Galeem and Dharkon wage war against each other using their captured spirits, with Galeem taking control of Mythra.
Mythra uses a female Corrin puppet fighter and is fought on the Gaur Plain's Battlefield form. She is fought along a Shulk puppet fighter representing Rex. During the battle, Corrin starts off with an enhanced Beam Sword, referencing Mythra's status as a Light Blade.
Names in other languages